Telomeres, the protein-DNA structures physically located on the ends of the eukaryotic organisms, are required for chromosome stability and are involved in chromosomal organization within the nucleus (See e.g., Zakian, Science 270:1601 [1995]; Blackburn and Gall, J. Mol. Biol., 120:33 [1978]; Oka et al., Gene 10:301 [1980]; and Klobutcher et al., Proc. Natl. Acad. Sci., 78:3015 [1981]). Telomeres are believed to be essential in such organisms as yeasts and probably most other eukaryotes, as they allow cells to distinguish intact from broken chromosomes, protect chromosomes from degradation, and act as substrates for novel replication mechanisms. Telomeres are generally replicated in a complex, cell cycle and developmentally regulated, manner by "telomerase," a telomere-specific DNA polymerase. However, telomerase-independent means for telomere maintenance have been described. In recent years, much attention has been focused on telomeres, as telomere loss has been associated with chromosomal changes such as those that occur in cancer and aging.
Telomeric DNA
In most organisms, telomeric DNA has been reported to consist of a tandem array of very simple sequences, which in many cases are short and precise. Typically, telomeres consist of simple repetitive sequences rich in G residues in the strand that runs 5' to 3' toward the chromosomal end. For example, telomeric DNA in Tetrahymena is comprised of sequence T.sub.2 G.sub.4, while in Oxytricha, the sequence is T.sub.4 G.sub.4, and in humans the sequence is T.sub.2 AG.sub.3 (See e.g., Zakian, Science 270:1601 [1995]; and Lingner et al., Genes Develop., 8:1984 [1994]). However, heterogenous telomeric sequences have been reported in some organisms (e.g., the sequence TG.sub.1-3 in Saccharomyces). In addition, the repeated telomeric sequence in some organisms is much longer, such as the 25 base pair sequence of Kluyveromyces lactis. Moreover, the telomeric structure of some organisms is completely different. For example, the telomeres of Drosophila are comprised of a transposable element (See, Biessman et al., Cell 61:663 [1990]; and F.-m Sheen and Levis, Proc. Natl. Acad. Sci., 91:12510 [1994]).
The telomeric DNA sequences of many organisms have been determined (See e.g., Zakian, Science 270:1601 [1995]). However, it has been noted that as more telomeric sequences become known, it is becoming increasingly difficult to identify even a loose consensus sequence to describe them (Zakian, supra). Furthermore, it is known that the average amount of telomeric DNA varies between organisms. For example, mice may have as many as 150 kb (kilobases) of telomeric DNA per telomere, while the telomeres of Oxytricha macronuclear DNA molecules are only 20 bp in length (Kipling and Cooke, Nature 347:400 [1990]; Starling et al., Nucleic Acids Res., 18:6881 [1990]; and Klobutcher et al., Proc. Natl. Acad. Sci., 78:3015 [1981]). Moreover, in most organisms, the amount of telomeric DNA fluctuates. For example, the amount of telomeric DNA at individual yeast telomeres in a wild-type strain may range from approximately 200 to 400 bp, with this amount of DNA increasing and decreasing stoichastically (Shampay and Blackburn, Proc. Natl. Acad. Sci., 85:534 [1988]). Heterogeneity and spontaneous changes in telomere length may reflect a complex balance between the processes involved in degradation and lengthening of telomeric tracts. In addition, genetic, nutritional and other factors may cause increases or decreases in telomeric length (Lustig and Petes, Natl. Acad. Sci., 83:1398 [1986]; and Sandell et al., Cell 91:12061 [1994]). The inherent heterogeneity of virtually all telomeric DNAs suggests that telomeres are not maintained via conventional replicative processes.
In addition to the telomeres themselves, the regions located adjacent to telomeres have been studied. For example, in most organisms, the sub-telomeric regions immediately internal to the simple repeats consist of middle repetitive sequences, designated as telomere-associated ("TA") DNA. These regions bear some similarity with the transposon telomeres of Drosophila. In Saccharomyces, two classes of TA elements, designated as "X" and "Y,"' have been described (Chan and Tye, Cell 33:563 [1983]). These elements may be found alone or in combination on most or all telomeres.
Telomeric Structural Proteins
Various structural proteins that interact with telomeric DNA have been described which are distinct from the protein components of the telomerase enzyme. Such structural proteins comprise the "telosome" of Saccharomyces chromosomes (Wright et al., Genes Develop., 6:197 [1992]) and of ciliate macronuclear DNA molecules (Gottschling and Cech, Cell 38:501 [1984]; and Blackburn and Chiou, Proc. Natl. Acad. Sci., 78:2263 [1981]). The telosome is a non-nucleosomal, but discrete chromatin structure that encompasses the entire terminal array of telomeric repeats. In Saccharomyces, the DNA adjacent to the telosome is packaged into nucleosomes. However, these nucleosomes are reported to differ from those in most other regions of the yeast genome, as they have features that are characteristic of transcriptionally inactive chromatin (Wright et al., Genes Develop., 6:197 [1992]; and Braunstein et al., Genes Develop., 7:592 [1993]). In mammals, most of the simple repeated telomeric DNA is packaged in closely spaced nucleosomes (Makarov et al., Cell 73:775 [1993]; and Tommerup et al., Mol. Cell. Biol., 14:5777 [1994]). However, the telomeric repeats located at the very ends of the human chromosomes are found in a telosome-like structure.
Telomere Replication
Complete replication of the ends of linear eukaryotic chromosomes presents special problems for conventional methods of DNA replication. For example, conventional DNA polymerases cannot begin DNA synthesis de novo, rather, they require RNA primers which are later removed during replication. In the case of telomeres, removal of the RNA primer from the lagging-strand end would necessarily leave a 5'-terminal gap, resulting in the loss of sequence if the parental telomere was blunt-ended (Watson, Nature New Biol., 239:197 [1972]; Olovnikov, J. Theor. Biol., 41:181 [1973]). However, the described telomeres have 3' overhangs (Klobutcher et al., Proc. Natl. Acad. Sci., 58:3015 [1981]; Henderson and Blackburn, Mol. Cell. Biol., 9:345 [1989]; and Wellinger et al., Cell 72:51 [1993]). For these molecules, it is possible that removal of the lagging-strand 5'-terminal RNA primer could regenerate the 3' overhang without loss of sequence on this side of the molecule. However, loss of sequence information on the leading-strand end would occur, because of the lack of a complementary strand to act as template in the synthesis of a 3' overhang (Zahler and Prescott, Nucleic Acids Res., 16:6953 [1988]; Lingner et al., Science 269:1533 [1995]).
Nonetheless, complete replication of the chromosomes must occur. While conventional DNA polymerases cannot accurately reproduce chromosomal DNA ends, specialized factors exist to ensure their complete replication. Telomerase is a key component in this process. Telomerase is a ribonucleoprotein (RNP) particle and polymerase that uses a portion of its internal RNA moiety as a template for telomere repeat DNA synthesis (Yu et al., Nature 344:126 [1990]; Singer and Gottschling, Science 266:404 [1994]; Autexier and Greider, Genes Develop., 8:563 [1994]; Gilley et al., Genes Develop., 9:2214 [1995]; McEachern and Blackburn, Nature 367:403 [1995]; Blackburn, Ann. Rev. Biochem., 61:113 [1992];. Greider, Ann. Rev. Biochem., 65:337 [1996]). The activity of this enzyme depends upon both its RNA and protein components to circumvent the problems presented by end replication by using RNA (i.e., as opposed to DNA) to template the synthesis of telomeric DNA. Telomerases extend the G strand of telomeric DNA. A combination of factors, including telomerase processivity, frequency of action at individual telomeres, and the rate of degradation of telomeric DNA, contribute to the size of the telomeres (i.e., whether they are lengthened, shortened, or maintained at a certain size). In vitro, telomerases may be extremely processive, with the Tetrahymena telomerase adding an average of approximately 500 bases to the G strand primer before dissociation of the enzyme (Greider, Mol. Cell. Biol., 114572 [1991]).
Importantly, telomere replication is regulated both by developmental and cell cycle factors. It has been hypothesized that aspects of telomere replication may act as signals in the cell cycle. For example, certain DNA structures or DNA-protein complex formations may act as a checkpoint to indicate that chromosomal replication has been completed (See e.g., Wellinger et al., Mol. Cell. Biol., 13:4057 [1993]). In addition, it has been observed that in humans, telomerase activity is not detectable in most somatic tissues, although it is detected in many tumors (Wellinger, supra). This telomere length may serve as a mitotic clock, which serves to limit the replication potential of cells in vivo and/or in vitro. What remains needed in the art is a method to study the role of telomeres and their replication in normal as well as abnormal cells (i.e., cancerous cells). An understanding of telomerase and its function is needed in order to develop means for use of telomerase as a target for cancer therapy or anti-aging processes.